Micro-concrete (MC) can be defined as a high performance cement-based material proportioned with micro-aggregates whose particle sizes range from about 0.5 nm to less than 1 mu m. The incorporation of micro-aggregates improve the particle packing density of the cementitious system, by this way the rheological and mechanical properties of MCs are enhanced. An experimental program has been conducted to investigate the effect of three types of micro-aggregates presenting three different particle size distributions (PSDs) on the admixture requirement, compressive and flexural strength development of MC. In order to compare the PSD of micro-aggregates and to find a correlation between fineness and admixture requirement, the concept of "Fineness Index" was described. Additionally, particle shape and surface morphology of micro-aggregates were analysed by using SEM images. Some conventional commands of image analysis were employed. Possible quantification methods of shape characteristics and surface roughness of micro-aggregates were proposed. The results showed that, in addition to PSD, particle shape and surface morphology of micro-aggregates should also be quantitatively determined. Success of characterization significantly depends on preparation of appropriate and representative SEM images and proper selection of methodology of analysis. (c) 2007 Elsevier Ltd. All rights reserved.